676 GARY N. CALKINS 



the edges of the peristome fuse is erroneous, while Biitschli 

 was right in stating that both margins of the peristome are free 

 (fig. 17). It thus happens that the adoral zone of one individual 

 overlies the line of fusion on one side, while the adoral zone 

 of membranelles of the other individual covers the line of fusion 

 on the opposite side. Fusion, furthermore, does not extend 

 posteriorly as far as the mouth but involves only about three- 

 quarters of the peristome, leaving the mouth and a large por- 

 tion of the undulating membrane free (fig. 14). B. undulans 

 is never very active but during the process of conjugation it is 

 even more quiescent than usual. The individuals remain united 

 for a period of from eighteen to twenty-four hours, rarely longer 

 than this, and finally separate. The process of separation 

 begins with the vacuolization of the zone of fusion (fig. 22). 

 The walls of the central vacuoles break down, leaving the two 

 cells connected at the anterior tip and at the posterior point of 

 fusion. The latter connection breaks first and the organisms 

 finally separate at the anterior tips (fig. 21). 



The conjugating population is distinctly smaller than the 

 non-conjugating, a fact easily noted by the eye. Statistical 

 confirmation of the size differences has been made by Miss 

 Watters, whose paper on the subject will shortly appear, and 

 she permits me to summarize her results to date as follows: 

 482 conjugating and 1089 non-conjugating individuals were 

 measured, the average length of the former being 110.08 mi- 

 crons, and of the latter (including ex-con jugants) 140.37 microns. 

 The longest conjugating individual was 156.25, non-conjugat- 

 ing 221.35 microns; the shortest conjugant was 85.93 microns, 

 non-conjugant 70.31 microns. There can be little doubt 

 that the shortest non-conjugant was an ex-conjugant, for these 

 are always minute. 



Stein believed that small forms with numerous nuclei were ex- 

 conjugants, but Biitschli regarded such forms as abnormal 

 specimens, not necessarily ex-conjugants. Under strictly nor- 

 mal conditions the nucleus may be multiple in B. undulans, 

 without indicating any connection with conjugation. The 

 smaller size of the suspected ex-conjugants and the evidences 



